Integrated Design Of Direct- Drive Linear Generators For...
Transcript of Integrated Design Of Direct- Drive Linear Generators For...
Integrated Design Of Direct-Drive Linear Generators For
Wave Energy Converters
A. S. McDonald, R. Crozier, S. Caraher, M. A. Mueller and J. P. Chick.Supergen Marine
Institute for Energy SystemsJoint Research Institute for Energy
Edinburgh Research Partnership in Engineering and MathematicsSchool of Engineering & Electronics
The University of Edinburgh
Agenda
� Introduction to Direct-Drive and Linear Generators
� Design approach
� Structural and magnetic design
� Bearing design
� Future Work
Direct Drive Linear Generators� Reduction of Failure Points� Potential for Maintenance
Free Operation� Potential for Submerged
Operation� Flexibility and Potential for
Improvement� Generators can be large and
heavy� Permanent magnets are
expensive� Large forces in the machine –
lots of structural material required
� No in-built energy storage
Fig 1: Mueller, M. A., Baker, N. J. (2005). "Direct drive electrical power take-off for offshore marine energy converters." Proceedings of the I MECH E Part A Journal of Power and Energy 219: 223-234.
Direct Drive Machines For Wave Energy
Linear PM Synchronous Machine (Double-Sided)
Linear Air-Cored PM Synchronous Machine
� Iron Core� High Power� Large Forces� Low Power Factor
� Air Core� Lower Power� Lower Forces� High Power Factor
Typical Design Approach
Wave FrequencyDistribution
WECmodel
Electricalmodel
Structuralmodel
Criterioncalculation
5 MW3 MW2 MW
Thermalmodel
Electrical DesignOptimisation
DesignCheck
FinalDesign
Improved Design Approach
Wave FrequencyDistribution
WECmodel
Generatormodel
Electricalmodel
Structuralmodel
Criterioncalculation
5 MW3 MW2 MW
Thermalmodel
DesignOptimisation
FinalDesign
Direct Drive Machines For Wave Energy
Linear PM Synchronous Machine (Double-Sided)
� Iron Core� High Power� Large Forces� Low Power Factor
Pictures from Polinder et al, “Linear PM Generator System for Wave EnergyConversion in the AWS”, IEEE transactions on energy Conversion, september 2004
Iron
Structural Steel magnet
Undesirable displacement
Pow
er P
rodu
cing
Mot
ion
Change In Forces With Displacement
Structural Design - Beams
� The forces must be withstood by some support structure
� The proposed structure is a series of beams running transverse to the direction of motion
� The I-Beam is chosen for its efficiency in its use of material – i.e. lower cost
� The beams are laid with no concern for the division by electrical pole
Iron-CoredI-Beam
Air-Cored
Impact on Capital Cost� The beam cross-section necessary to withstand the forces can
be determined by iteratively determining the forces on the beam as it deflects
� The choice of I-Beam sections can then be made from the following standards: BS4, Continental IPA, IPEA and IPN
276309
276243
276177
224299
224233
224167
172289
172223
172157
15
237284
237218
237152
185274
185208
185142
132264
132198
132132
10
198258
198192
198127
146248
146182
146117
94238
94172
94106
5
30, 930, 630, 320, 920, 620, 310, 910, 610, 3Copper
Cost €/kg
Magnet, Iron Cost €/kg
Top figure in each table cell is the cost of the ai r-cored machine, the second lower figure is the cost of the iron-cored machine.
237152
Bearings
� The structural analysis assumes that the material between the translator and stator is infinitely stiff
� In reality a bearing system must be designed to approximate this over a given lifetime
� This presents many challenges due to the harsh environment and high loadings in the machine
Internal Loads External Loads
� Wave loads were approximated using WAMIT giving a force of 177kN for a submerged WEC in depth of 30m and high frequency seas of 1.5Hz.
Net Force seen by the bearings
Airgap closing force in iron-cored machine
WEC length 21m,Ø9m.
Forces on machines:227kN on Iron Cored.177kN on Air Cored.
Forces to expect are higher the closer the WEC is to the surface.
Source: Gregory Payne
Bearing Options� Rolling contact , Thrust and Angular Ball Bearings.
� Need oil as lubricant – degrades over time� Lifespan calc’s can give years of operation� Reaction to shock loading is limited� Lubrication intervals -max yearly
� Linear Slides and Profile rails. � Maintenance and constant operation problems
� LBCT ,greasing every 800hrs, long life under low load.� LLHRC, cleaning stroke daily, life of ~600km.
� Sealing for linear rails difficult� Misalignment from rail over a long stroke.
� Water lubricated systems - Promising� Fluid film options� Contact sliding bearings from marine suitable polymers
� No seals, no pollution, fluid film reduces surface contact
(SKF.com)
SXL (Thordon Inc)
T814 (Tenmant Ltd.)
Design ConsiderationsAir- Cored
Machine LoadsIron Cored
Machine Loads
Bearing loads
Sliding Contact
Rolling Contact
E.H.LH.L B.L
Life span Prediction
f
Wave LoadingOn Machine
Hydrostatic/Hydrodynamic
wear δT
Hydrodynamic lubrication
- Full film between parts in contact
Elasto –Hydrodynamic-lubrication- v. thin film between parts in contact
Boundary lubrication- Asperite contact
5000hrs travel8000km/ yr
Bearings for flooded generators
Industrial Machinery� IGUS,
� GGB, Glacier Garlock Bearings
� Dry or Water Lubricated� IGLIDUR UW, f 0.12 - 0.5 or 0.04� GGB DUB, f 0.02 - 0.12
Marine applications� Thordon SXL, propeller shafts� Tenmat T814, hydroelectric dams
� Benefits in Linear applications� Low abrasive wear, corrosion resistant� Low modulus of elasticity, ability to handle shock loads� High Load ratings – up to 40MPa
SXL
(Thordon Inc)
T814(Tenmant Ltd.)
Bearing Summary � For fully non-contact operation:
� Hydrostatics or Magnetic Levitation� Active systems with higher power demand.� Additional control needed, fluid flow or field control � Length of translator stroke makes running clearance hard to maintain
� Contact back-up system:� Water lubricated polymer materials.� Testing due to start on these materials to verify low wear claims.
� Options for Iron cored Machine are limited � Hybrid of non-contact bearings and a back up contact system
� Air Cored Machine more achievable� Water lubricated contact bearings are a promising option
Future Work
� Design and integrate thermal models
� Develop optimisation method� Have investigated Genetic Algorithms as a promising
option
� Completion of a test rig and physical testing of bearing materials
� Systemise bearing design for inclusion in cost and feasibility models of linear generators
The Following Slides are for Reference if Necessary
Hydrostatic Bearings
� Benefits-� Low friction
� f 10-3 – 10-6
� Low maintenance� Cooling effects on coils � Less stringent sealing if
flooded with water
� Difficulties-� High attractive forces in machines� Materials/methods for construction� Use of seawater
� Corrosion� Low Viscosity
Important Relationships
• Pad load
• Pumping Power
3zwh
Qγα
3
2
h
QH p
γ∝
“two moving surfaces separated by thin film of pres surized liquid “
Suitability for D-D Machines� Water provides thin fluid films
� Again – low viscosity of water lessens the achievable effect.
� Machining surfaces to a fine finish over lengths required is costly
� Power demand increases hugely with increased film.� Lower load carrying capability in comparison to oil.
� Reliability with water in an iron cored machine is low� Surface Contact may be expected due to small films.
� Operation in air cored machines is more promising� Power demand is lower.
Pressure Drop across pad
Schematic of Hydrostatic system
PM synchronous Linear GeneratorOperating considerations:
• Wave amplitude, 4 m
• Wave period, 10s
•6 strokes/min
• Constant sinusoidal operation
• Distance to traverse per year > 7,033km
(inc. redundancy of 40%)
External Forces
Challenges in Hydrostatic operation
Problems with sea water;� Corrosion & clogging if not filtered
� Clearances are extremely low� Operating range 12 - 50 µm in
comparison to oil ~150 - 300µm� Precision Machining methods for
surfaces add cost
� Implementing bearings into design means� Creating compliant translator� Single pad & opposed pad fluid
control using minimal pumps
Conclusion on Pad geometry
• Circular pads are favoured geometry
•Larger operating clearances
•Less power loss at optimum
•Less edge effects on flow
5..
If oil is used;
•Extend the clearance
•Operating points for B4 , ro - 0.15m•hopt ~ 0.5mm•Hp 7.5 W/pad
•Reliance on tight seals•Chance of leakage to sea
•Cooling benefits not as great
Bearing placement
Pad Geometry Investigation -Rectangular Pads
� Pad stiffness highest at low clearance
� Pads 2,3,4 with x - 0.049m most potential� Lower leakage at larger pad sizes� Lower operating clearances
Operation with seawater as lubricant
Pad 3 chosen for further analysis35 pads/sidePr = 0.304 MPaPressure to provide “lift”
Operation with oil as lubricant
Pad Geometry Investigation -Circular pads
� As before stiffness highest at low clearance
� Most promise - Pads ØD 0.15 and 0.12 with ri/ro 0.5� Lower leakage at smaller pad sizes� Lower operating clearances
Operation with oil as lubricantOperation with seawater as lubricant